OV COHESION. ' 619 



equilibrium; and if any cause should tend to separate them, or to bring them 

 nearer together, they would resist it, in either case, with a force proportional 

 to the degree of extension or compression. The distance, at which the force 

 of cohesion commences, is not the same for all kinds of matter, nor even for 

 the same substance at different temperatures; it is smaller for vapours of all 

 kinds, in proportion as their temperature is higher, the cohesion itself being 

 also smaller. If the experiments on the density of steam have been correct, 

 it follows that the force of repulsion must increase more rapidly than the 

 distances diminish, for the elasticity of water is nearly ten times as great as 

 that which would be inferred from the compression of steam into a substance 

 of equal density : this supposition agrees also with the eicperiments on the 

 mean density of the earth,- which is probably not so great as it would be if 

 the force of repulsion increased in the simple ratio of the density. The law 

 of repulsion appears also to l)e in some degree modified by the effect of heat, 

 which increases its force at greater distances more considerably than at smaller. 

 It appears indeed, from the diminution of the elasticity of a spring by heat- 

 ing it, that the repulsive force of the particles of bodies at very small dis- 

 tances is even diminished by heat, unless the force be again supposed to 

 decrease much more rapidly than the distance diminishes: thus the diminu- 

 tion of the elasticity of iron by heat is about thirty times as great as the increase 

 of the distance of its particles; so that the original repulsive force must 

 probably be somewhat diminished, although less tlian the cohesive force. 

 At greater distances, however, the force of repulsion is certainly increased ; for 

 the elasticity of vapours and gases of all kinds is evidently greater as the tem- 

 perature is higher. (Plate XXXIX. Fig. 530.) 



The cohesion of two or more particles of matter to each other does not 

 interfere with their power of repelling other particles situated in a different 

 direction: thus, two pieces of glass require to be brought together with con- 

 siderable force, and generally with some friction, before they can begin to 

 cohere; and a small drop of water, falling lightly on the surface of a pond, may 

 remain for some instants without coming into perfect contact with it; the same 

 circumstance is also still more observable in spirit of wine a little warmed. 



The first and simplest effect of cohesion is to produce liquidity. That 

 all liquids possess some cohesion, is very obvious, from their tendency to 

 assume a spherical form when they are sufhciently detached from other subr- 



